Get those sparkplugs checked by someone with a plug tester and an ear
for the job. We have had considerable roughness caused by bad plugs
recently.
We used to use nothing but Auburn plugs until Champion bought them
up and shut them down, and had to use Champs even though they fail
internally after only a couple hundred hours. When Unison brought out
the Autolite plugs, which are based on the old Auburn design, we got
good service until lately when we've been finding the plugs' internal
resistance going up and causing arcing in the connector barrel. You
have to know what to listen for when testing them. The spark in the
cylinder gets weak and much more sensitive to mixture inconsistencies
and changes.
When is someone going to bring out a reliable aircraft engine
plug?

Dan